What was once a no-man’s-land is about to become a pharmaceutical battleground.
The Food and Drug Administration approved a second new weight-loss drug on Tuesday, expanding the range of medical options for the one-third of American adults who are obese.
The drug, Qsymia, offered the greatest weight loss in clinical trials of the three diet drugs that have been considered by the F.D.A. in recent years. But it also raises the risk of birth defects if used by pregnant women and can cause elevated heart rates and cognitive problems.
Qsymia, previously known as Qnexa, was developed by Vivus of Mountain View, Calif. It is the second new drug for obesity approved in the last month, after Belviq from Arena Pharmaceuticals.
Before that, no new prescription weight-loss drugs had been approved in 13 years, and the one drug approved for long-term use, Roche’s Xenical, was rarely prescribed.
The F.D.A. has been cautious about approving such drugs because of safety problems, most notably the heart valve damage caused by a drug used in the fen-phen diet pill combination.
Both Qsymia and Belviq were rejected by the F.D.A. in 2010 but the manufacturers came back this year with new data. In February, an F.D.A. advisory committee voted to approve Qsymia (pronounced kyoo sim EE uh).
A third drug, Orexigen Therapeutics’ Contrave, was also rejected and is now being tested in a new clinical trial.
Vivus said Qsymia is expected to be available in the fourth quarter. Peter Tam, the company’s president, declined to disclose the price, although he suggested it would be similar to the price of diabetes drugs. Some newer diabetes drugs cost about $6 a pill.
Qsymia is a combination of two existing drugs, one of which is phentermine, an appetite suppressant that is the component of the fen-phen combination that remained on the market. The other is topiramate, an epilepsy and migraine drug.
Some obesity specialists have already been prescribing each component of Qsymia separately.
The approval sets up a potential marketing battle.
Vivus provided better average weight loss. In late-stage clinical trials, patients on the middle dose of Qsymia lost an average of 8.4 percent of their weight after one year. Those on the high dose lost 10.6 percent. By comparison, those on Arena’s Belviq lost an average of 5.8 percent of their weight after one year.
Still, some doctors might judge Belviq to be less risky. And it will be sold by a fairly big drug company, Japan’s Eisai, while Vivus is going it alone on Qsymia.
Simos Simeonidis, an analyst at Cowen & Company, said that while he thought Qsymia would do better, there was room for both drugs.
“The majority of obese patients wanting to try drug therapy will end up seeing both drugs, regardless of which one they take first,” he wrote in a note Tuesday.
Indeed, the biggest problems for Vivus and Arena might not be each other but the difficulties in selling obesity drugs. While many people might try such drugs, they rarely stay on them for more than a few months. In addition, insurance companies do not often pay for such drugs.
The F.D.A. suggested various safety measures to reduce the risk of problems. It said patients should start on a middle dose of the drug and should consider a high dose only if they do not lose 3 percent of their body weight after 12 weeks. If patients do not lose 5 percent of weight after 12 weeks on the high dose, they should discontinue the drug.
The agency also said the heart rates of people taking the drug should be regularly monitored. Women of childbearing age should have a negative pregnancy test before starting Qsymia and every month while using the drug.
By ANDREW POLLACK/The New York Times